1. Field of the Invention
The present invention is broadly concerned with a greatly improved encoding device which, in preferred forms, directly generates digitalized output signals and provides reliable, long lived operation at minimum cost. More particularly, it is concerned with such an encoding device especially adapted for use in an impact-type keyboard to produce signals that are of extremely short duration (on the order of milliseconds or even microseconds) in order to afford a considerable degree of N-key rollover protection while at the same time eliminating the need for precision manufacture or strict tolerance limits in the components of the device.
Attention is directed to U.S. Pat. No. 4,359,612, issued Nov. 16, 1982, entitled "Universal Keyboard and Method of Producing Same", and U.S. Pat. No. 4,359,613, issued Nov. 16, 1982, entitled "Molded Keyboard and Method of Fabricating Same." Both of these patents are hereby incorporated by reference into the instant application.
2. Description of the Prior Art
A wide variety of keyboards have been proposed in the past for use in connection with typewriters, computer input stations, and numerous other devices. In the present state of the art, there are basically three types of keyboards. In one variety, electronic output in the form of electrically encoded signals to a companion or remote device is employed. In another type of keyboard, mechanical output movements are used which trip or activate leverages or linkages in either totally mechanical machines (e.g., manual typewriters) or electric machines such as electric typewriters. The principal distinguishing feature between the two types of keyboards is the form of output, i.e., mechanical movement or electric signal. A third type of general keyboard construction can be thought of as a hybrid between the electronic and mechanical units. In this form, a mechanically induced movement is read electronically by one of various kinds of transducers, and the reader outputs the detected movement in the form of signals of an electronic nature.
In keeping with the diversity of keyboard designs, a number of keyboard output devices have been suggested, and such can be broadly classed as either generating mechanical output movements or electronic signals. The latter type of output device is generally characterized by complicated logic circuitry and associated structure which is capable of sensing the depression of a particular key and, in response to such sensing, generating an electronic output signal, typically in the form of a digitalized signal. Additionally, provision must be made by scanning or other techniques to prevent false signals or ambiguities by virtue of problems associated with N-key holddowns and key teasing. It will of course be appreciated that such expedients add considerably to the cost and complexity of an output device, and are therefore deficient for these reasons.
U.S. Pat. No. 3,353,038 to Mason et al. describes a keyboard wherein use is made of elongated, vibratory cantilevers mounted adjacent each key arm. A piezo-electric crystal element is mounted in contact with each vibratory cantilever in order to generate an output signal for the key. In operation, depression of a key serves, through an intermediate magnet, to draw the associated cantilever downwardly until a stop is reached; at this point the cantilever is disengaged from the magnet, and begins to vibrate, thus generating an output signal through the piezo-electric crystal. A similar construction is disclosed in U.S. Pat. No. 3,725,908 to Brisebarre et al. In this unit, an elongated cantilever is actuated upon key depression in order to impact-engage an underlying piezo-electric crystal.
Both the above mentioned constructions are incapable of directly generating a digitalized keyboard output. That is to say, each cantilever actuates one and one only piezo-electric crystal, thus necessitating logic circuitry or the like for the purpose of encoding the respective signals received from the keys as they are depressed.
U.S. Pat. No. 4,258,356 discloses a keyboard having an encoding apparatus in which strikers are actuated by the keys of a keyboard. These strikers engage one or more parallel acoustic bars, in order to generate acoustic energy which is ultimately transduced as an encoded signal. A significant problem associated with keyboards of the type described in U.S. Pat. No. 4,258,356 stems from the fact that the strikers are, at rest, in contact with the associated acoustic bars. This not only presents formidable difficulties from the standpoint of tolerances and machining requirements, in assuring proper, multiple contact points, but can also give rise to cross vibrations and false signals. That is to say, vibrations induced in one bar can be transmitted through downstream strikers in engagement with the bar and other bars so that the transducer apparatus coupled to the other bars senses a vibration when, in reality, such other bars have not actually been struck. In addition, the acoustic output system must be isolated from accidental noise from sources other than the keyboard, else false signals will be generated.
In short, there is real and heretofore unsatisfied need in the art for a keyboard output device which is rugged, compact, low cost, free of electronic encoding circuitry, and which gives directly generated, digitalized output signals.